1. Field of the Invention
The present invention relates to a quadrature demodulation device that performs quadrature detection on a reception signal obtained by receiving a radio wave from, for example, a radio frequency identification (RFID) tag.
2. Description of the Related Art
In recent years, radio frequency identification (RFID) tags have been utilized in various fields. The RFID tag is a kind of responder that makes short distance radio communications with an interrogator. The interrogator transmits a carrier modulated for “interrogation” and a succeeding radio wave that is an unmodulated carrier. The RFID tag responds to the interrogation from the interrogator and performs backscatter modulation to superimpose reply data on the unmodulated carrier. The RFID tag then transmits a radio wave resulting from the modulation to the interrogator. The reply data is a data signal comprising a synchronization signal portion and a succeeding data signal portion which are encoded at a predetermined bit rate. The synchronization signal portion contains a preamble having a particular transition pattern that can be detected by bit synchronization, and the succeeding data signal portion contains data of an identification code.
The interrogator receives the radio wave transmitted by the RFID tag as a reception signal. The interrogator then performs quadrature detection on the reception signal to reproduce the reply data. The quadrature detection is a scheme that enables direct conversion of a reception signal from an antenna into a base band. The quadrature detection involves mixing a local carrier signal having a frequency set equal to the carrier frequency of the reception signal, with the reception signal to generate an in-phase (I) signal with the base band and mixing a signal obtained by shifting the phase of the local carrier signal by 90 degrees with the reception signal to generate a quadrature-phase (Q) signal with the base band. The amplitude of the I signal and the amplitude of the Q signal depend on the difference between the phase of the reception signal and the phase of the local carrier signal. The amplitude of the Q signal is minimized by maximizing the amplitude of the I signal. The amplitude of the Q signal is maximized by minimizing the amplitude of the I signal. Each of the I and Q signals contains a signal component equivalent to the above reply data together with a noise component involved in radio transmission.
The following interrogator has been known. The interrogator compares each of the I and Q signals with a particular transition pattern provided for preamble detection. Upon detecting that both the I and Q signals have a preamble matching the particular transition pattern, the interrogator decodes the I and Q signals in order to obtain data succeeding the preamble (for example, see U.S. Pat. No. 6,501,807 B1).
The preamble may be incorrectly detected. Consequently, the interrogator described in the above publication is configured to avoid performing the decoding if the interrogator has failed to detect the preamble in one of the I and Q signals. Thus, if only the preamble of the I signal is detected, even if the I signal can be correctly decoded, the I signal is discarded without being decoded.